Sahih Bukhari Volume 007, Book 062, Hadith Number 001.
Narrated By Anas bin Malik : A group of three men came to the houses of the wives of the Prophet asking how the Prophet worshipped (Allah), and when they were informed about that, they considered their worship insufficient and said, "Where are we from the Prophet as his past and future sins have been forgiven." Then one of them said, "I will offer the prayer throughout the night forever." The other said, "I will fast throughout the year and will not break my fast." The third said, "I will keep away from the women and will not marry forever." Allah's Apostle came to them and said, "Are you the same people who said so-and-so? By Allah, I am more submissive to Allah and more afraid of Him than you; yet I fast and break my fast, I do sleep and I also marry women. So he who does not follow my tradition in religion, is not from me (not one of my followers)."